A brushless direct current (BLDC) motor may prevent friction and wear which are disadvantages of the existing DC motor and have relatively high efficiency. Therefore, recently, hybrid cars tend to adopt the BLDC motor as a motor for rotating a cooling fan.
The BLDC motor is a motor that does not have a brush and a commutator necessary for a DC motor but has an electronic commutation mechanism installed therein. Among the BLDC motors, an inner-rotor type BLDC motor includes a rotor and a stator, in which the rotor whose center is provided with a permanent magnet rotates and the stator whose circumference is wound with a drive coil is fixed. That is, the stator whose outer side is wound with the drive coil is fixed and the rotor whose inner side is provided with the permanent magnet rotates.
As illustrated in FIGS. 1 and 2, the conventional inner-rotor type BLDC motor 1 includes a stator 30 fixed to an inner side of the housing 10 and rotors 20 disposed therein at a predetermined interval. The stator 30 which has a ring shape is provided with a plurality of teeth 31 which are protruded and formed radially inwardly. An upper part and a lower part of the stator 30 are coupled to insulators 40 in a form in which the insulators 40 surround the stator 30 to insulate the stator 30, and a drive coil 60 is wound around the teeth 31 of the stator 30 insulated by the insulator 40. In addition, a plurality of permanent magnets 21 which are arranged to be spaced apart from each other along a circumferential direction are coupled to the rotor 20.
In this case, the stator 30 is provided with a hall sensor 50 which detects a magnetic field generated from the rotor 20 to be able to determine a position of the rotating rotor 20, in which three hall sensors 50 are disposed to be able to detect three hall signals formed by the magnetic field generated from the rotor and having a phase difference of 120°.
The hall sensor 50 is a sensor which is operated by being applied with the magnetic field generated from the rotor 20. As illustrated in FIG. 2, an overhang structure in which an upper end of a core of the rotor 20 is disposed above an upper end of a core of the stator 30 is formed, and thus the hall sensor 50 is disposed to detect a change in the magnetic field in response to the rotation of the rotor 20.
Here, the hall sensor detects the change in the magnetic field at the time of the rotation of the rotor to identify a position signal of the rotor. Meanwhile, the hall sensor does not often accurately detect the change in the magnetic field to fail to identify the position information of the rotor, and therefore the motor may not be accurately controlled or even the driving of the driving may stop.
That is, if the overhang part of the rotor is not sufficient, the magnetic flux generated from the permanent magnet of the rotor is not sufficiently transmitted to the hall sensor, such that the hall sensor may not accurately detect the change in the magnetic field.